The present invention relates to a pack of heat transfer plates for use in heat exchangers, preferably in rotary regenerative air preheaters.
Heat transfer plates which each have corrugations formed by S-shaped double ridges on respective plates and intermediate flat or slightly undulated plate portions are known, for instance, from British Patent Specification Nos. 1,335,205 and 1,252,319, respectively. These known plates have been found to possess optimum values with regard to heat transfer characteristics and low pressure drop conditions. When stacked in wedge-shaped packs in regenerative air preheaters in particular, the advantages afforded by those known heat transfer plates are utilized fully in a particularly beneficial manner, since it has been found that such packs are less liable to become blocked by so-called soot and other particulate solids present in flue gases than are other types of heat transfer plates used for a similar purpose.
The known packs of plates are produced in accordance with three mutually different methods. According to the first of these methods, a strip of sheet metal is first rolled between two profiled rolls and therewith given the intended profiled shape, whereafter the thus profiled metal strip is cut progressively into a large number of pieces, the size of which represents, for instance, the smallest or narrowest dimension of a wedge-shaped pack of plates, whereafter the profiled metal strip is again cut into an equal number of plates, the size of which represents the next smallest dimension of the pack, and so on until plates having the largest desired dimension have been cut from said strip of profiled sheet metal. In this way there is produced a store of plates having the various requisite dimensions from which the plates required can be collected in sequence, with successively increasing or decreasing dimensions and the plates stacked upon each other with the double ridges of mutually adjacent plates intersecting one another, i.e. subsequent to turning each alternate plate through 90.degree., to form a wedge-shaped pack of plates. The flow channels defined by mutually contiguous plates will then have the pattern illustrated in the above mentioned British patent specifications.
According to the second of these three known methods, respective strips of sheet metal are rolled simultaneously in separate roll stands which are mutually so arranged that when the sheet metal strips are fed out from respective roll pairs in superposed relationship, the double ridges of respective strips will intersect one another in the manner illustrated, for example, in British Patent Specification No. 1,401,621. This method includes cutting both of the sheet metal strips simultaneously into smaller pieces while successively changing the plate dimensions after each cutting operation, so that the pieces cut from the strips can be stacked immediately in the form of a wedge-shaped body, whereafter the procedure is repeated for the next pack of plates in line.
The third of the aforesaid three known methods can be said to comprise a combination of the first and the second methods. In the case of the third method, there is used only one roll stand, and subsequent to dividing the metal strip into smaller pieces, or plates, the plates are formed into packs, although it is necessary in this case to turn each alternate plate subsequent to cutting said plate from the strip, so that the double ridges on mutually adjacent or contiguous plates will intersect one another.
The sole drawback exhibited by these known plates is that some difficulty is experienced in effectively blowing away so-called soot and products of corrosion, since the soot blowing jets are disintegrated in the flow channels between the plates by the obliquely extending double ridges. As a result the channels defined by mutually adjacent or contiguous plates may become partially blocked sporadically, which may necessitate shutting down the heat exchange system in order to clean the air preheater.
For this reason this type of heat transfer plate has not been accepted in some countries, despite being available for seventeen years, while in other countries the plate has been accepted on the merits of its high heat transfer performance and has been used in conjunction with auxiliary solutions for improving the ejection of so-called soot and other solids by blowing. One such solution has involved dividing the heat transfer plates into at least two parts in the direction of medium flow with an empty space between said two parts, so that so-called soot and other solids can be blown towards the empty space, from both ends of the plates. This solution is not an ideal solution, however, either from the aspect of blowing the plates clean of soot or from the aspect of space.
The object of the present invention is to improve the flow pattern in this type of plate pack, such as to prevent blocking of the channels between adjacent plates, either partially or completely, in a more effective manner.